SF OKs review of cracked Millennium window, safety...

1of3Cracked window in the Millennium Tower.Photo: San Francisco Department of Building Inspection

2of3Pedestrians walk beneath a scaffolding constructed in front of the lobby entrance to the Millennium Tower at 301 Mission Street last month after a window above was found to be cracked.Photo: Lea Suzuki / The Chronicle

3of3A crack in a window at the Millennium Tower was caused by an “exterior impact,” engineers say.Photo: San Francisco Department of Building Inspection

A Stanford University engineering professor working with San Francisco’s Department of Building Inspection concluded there’s “no reason” to further investigate what caused a window to crack on the 36th floor of the Millennium Tower.

Professor Greg Deierlein’s conclusions were presented in a letter that Building Inspection sent Thursday to the tower’s management. The department asked Deierlein to review the findings of a forensic report on the cracked window that found no evidence that the issue was linked to the building’s ongoing sinking and tilting.

The report, conducted by the engineering firm Allana Buick & Bers on behalf of the tower’s managers, blamed the crack on an “exterior impact,” but gave no indication as to what might have struck the window.

Deierlein found no reason to question the report’s findings.

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“While it’s fair to say DBI accepts Professor Deierlein’s findings per his review of the newly issued AB&B report, we understand that in order for (Millennium’s management) to complete its investigation of the cracked glass” the window would have to be removed and subjected to additional analysis, Department of Building Inspection spokesman William Strawn said.

The crack was discovered about 2:30 a.m. on Sept. 1 after the homeowner in unit 36B awoke to a “massive” banging sound.

“This sound was so massive in scale, it sounded like a huge, thick vault door had just slammed shut,” the homeowner said, describing the event to the Allana engineering firm. For the next 30 minutes, the homeowner described hearing what sounded like the glass splintering in the window.

The cracked window prompted widespread speculation about whether it was related to the tower’s sinking and tilting. Since it opened in 2009, Millennium Tower has sunk by around 18 inches and is tilting to one side.

Over concern for public safety, Building Inspection ordered the tower’s managers to tape up the window and install overhead scaffolding to prevent glass from falling onto the Mission and Fremont street sidewalks.

The tower’s managers indicated they were preparing to remove the scaffolding on the recommendation of the engineering firm’s report. But Building Inspection ordered the scaffolding to be left intact until the window is replaced, which must be completed by Nov. 18. The building’s management said last month the replacement glass had to be ordered from Shanghai, so the replacement would probably take place in mid-November.

The Department of Building Inspection also mandated that the tower’s management complete a “visual inspection” of all similar windows in the 416-unit tower by Dec. 21.

Dominic Fracassa covers San Francisco City Hall for The Chronicle. He previously worked as a reporter and editor for the Daily Journal, a legal affairs newspaper. He started in news in his home state of Michigan, where he worked as a news director of 103.9 WLEN.